If we were having this discussion in the late 1920s or early 1930s, most of us would probably be fans of the Kellogg-Briand Pact outlawing war. Not so today, obviously. But back in the 1930s, as Samuel Huntington points out, American academia put the emphasis “almost entirely upon the questions of form and structure studied in courses in international law and international organization,” aimed at vindicating “world organization.” After the catastrophic failure of such institutions to ward off fascism and global war, the American study of international relations, as taught in law schools and public policy schools, has embraced a postwar disenchantment brilliantly championed by the great realist Hans Morgenthau. Huntington writes, “By the late 1940’s, however, American writers were vying with each other in denouncing the moralism, legalism, utopianism, Wilsonism, and sentimentalism of the American diplomatic past.”

So Oona Hathaway and Scott Shapiro know perfectly well that they’re making a bold claim that the Kellogg-Briand Pact represents “among the most transformative events of human history.” Their new book is cogent and provocative, a gripping study of the diplomats and thinkers who pressed for the outlawry of war, but obviously headed for controversy from critics wary of Whiggish history.

Oona and Scott’s point in The Internationalists isn’t that Kellogg-Briand ended war, but that it ushered in a new legal order in which war is no longer a legitimate tool for righting wrongs, imposing a restraint on leaders contemplating war. They’re well aware that the Japanese invasion of Manchuria in 1931 came not long after the fancy signing ceremony in Paris for the Kellogg-Briand treaty in 1928. To their own lists of wars since Paris, you could add the 1962 war between India and China, the Sino-Soviet border conflict, the Vietnamese invasion of Cambodia, and Vladimir Putin’s annexation of Crimea, which Condoleezza Rice recently called “perhaps the greatest affront to the law-based international order in Europe since World War II.”

Oona and Scott might well point to Rice’s words as evidence that powerful decision-makers stand against aggression on grounds of law and world order. Their argument revolves around a legal shift which causes a cognitive shift in the minds of important leaders. At the same time, they insist upon the role of powerful states in maintaining what they see as a new legal order; their fear, appropriate to these dismal days of Trump, is that the United States will withdraw from that role.

The Internationalists is fundamentally, in Oona and Scott’s words, “a work of intellectual history”—not international law, not international relations, not political science, not diplomatic history, although it obviously contains elements of all of those. It’s about how “ideas matter” (their italics), as carried into practice by Robert Jackson, Hersch Lauterpacht, Sumner Welles, Nishi Amane, and others who influenced and advanced the development of international norms and law worldwide. This is another reason why some realists might not warm to it: neorealist theories of international relations are concerned with the structure of the system, dismissing as reductionist the internal workings of states, including ideas held by individuals within them.

Here’s what’s not controversial: it’s well known that there has been a steep decline in interstate war in the decades since World War II. This argument has been powerfully made by John Mueller (who says that war is not a necessity like breathing and eating, but more like obsolete social practices such as dueling and slavery), Richard Rosecrance, and John Lewis Gaddis, who has famously called the absence of major war among the great powers since 1945 “the Long Peace.” But this good news is usually attributed to, variously, a spreading zone of peace among liberal democracies, nuclear deterrence, the increasing destructiveness of war among industrialized states, declining advantages to territorial conquest, delegitimization of imperialism, economic interdependence, and so on. In his fascinating book The Better Angels of Our Nature, the psychologist Steven Pinker argues that the historical decline of violence (not just interstate war, but violence itself) is due to the ascendancy of our empathy, self-control, moral norms, and reason, exemplified by, among other things, the “Rights Revolutions” after World War II. For Mueller, in The Remnants of War, World War I was the crucial moment when people in the developed world got disillusioned with the value and efficacy of war, reinforced after the “spectacular anachronism” of World War II, which he blames almost entirely on Hitler. Ethical and legal constraints are often seen as part of the explanation for the decline of war, but not in the starring role given by Oona and Scott, and without the emphasis on Kellogg-Briand.

What are the observable implications of Oona and Scott’s argument? How can you measure mental change in the minds of policymakers? They do a great job of describing the frenetic activism of Lauterpacht and the others, the individuals whom political scientists would call norm entrepreneurs. They have an engrossing discussion of Nishi, the influential Japanese philosopher who introduced Western international law to a Japanese readership. The next step would be showing how the norm cascades outward after 1928 from their norm entrepreneurs into a wider world.

One way to observe norms in operation is to comb through the secret innermost records of government, to see the hidden debates and deliberations which result in policy. While international lawyers will sometimes point to public justifications to see legal rationales at work, political scientists will dig into private debates for evidence (in dreadful social science jargon, process-tracing). If Oona and Scott are right, we ought to see references in internal deliberations to Kellogg-Briand, or at least to its norms. Since it’s a new norm, on their account, it’s unlikely that its contents would be so internalized that its impact would go unspoken, or unchallenged.

To look for more such evidence, I did a little hunting in what seemed like a promising place: the secret diary of Kido Koichi, the closest aide to Emperor Hirohito, a deft player in the machinations of the Japanese court and government. This journal was used by the Allied prosecution at the Tokyo war crimes tribunal in 1946-48 as one of their crucial pieces of evidence. Kido was distinctly nationalistic but also suspicious of the expansionists in the domineering army, so would be more likely to have taken note of Kellogg-Briand than militarists like Tojo Hideki.

So what did Kido have to say about Kellogg-Briand during the Manchuria crisis in 1931? Zilch. He wrote neither about it, nor its norms, nor the reputational consequences for violating them. Instead, he privately worried about the renegade Japanese army, coup attempts, and the maneuverings of the imperial court. He (and other civilian politicians) did fret about the prospect of punishment from the League of Nations, or having to withdraw from the League, and he paid some attention to a U.S. statement by Henry Stimson that it could not accept the legality of any changes made by force in Manchuria. Kido feared that a delay in resolving the Manchurian crisis would make it harder to bring the rogue army under control, but not that Japan was violating its treaty commitment or trampling a legal norm against aggression.

Oona and Scott count Stimson’s views as evidence for their argument, and they’re right to classify militarist Japan as a country which believed in war as a tool of statecraft. Of course, this Kido inquiry was just one quick test, what social scientists call a plausibility probe. But if Oona and Scott are looking for more evidence which would silence critics, this could be one way to do it—except, obviously, with results that are the opposite of what was found in this small check. The Imperial Japanese view of the Kellogg-Briand Pact was closer to Mussolini’s, who signed it but called it “so sublime that it might be called transcendental.”